Unlike non-rechargeable batteries, rechargeable batteries may be recharged. Non-aqueous electrolyte rechargeable batteries with high energy density have recently been developed as high power rechargeable batteries.
Lower power batteries in which battery cells are made into a battery pack are used as a power source for various portable electronic devices such as cellular phones, laptop computers, and camcorders. Larger battery packs which are formed by connecting several to tens of rechargeable battery cells may be suitable for motor driven devices such as electric vehicles.
Rechargeable batteries may be classified into different categories based on their external shape, for example pouch-type, cylindrical, and prismatic shapes. The pouch-type case battery has been used in slim and light-weight portable electronic devices due to its flexible shape and light weight.
Unlike cylindrical or prismatic rechargeable batteries with cases that are formed of a relatively thick metal, pouch-type rechargeable batteries have a case formed of a thin metal film with insulating films attached at both sides thereof so that the battery is flexible. The pouch-type case has a space for housing the electrode assembly, and closing and sealing parts for heat-sealing along the edge of the space.
The conventional rechargeable battery may explode due to gas generation when its internal voltage increases due to overcharging. Particularly, a lithium ion battery may have increased internal pressure caused by released gas, such as carbon dioxide, carbon monoxide, and the like, when a liquid electrolyte is decomposed due to an overcharge. In addition, the battery may catch fire when the internal temperature of the battery increases due to over-current caused by an over-discharge or over-charge.
In general, a rechargeable battery can be used safely only when temperature requirements both at charge and during operation are met. However, when a temperature at battery charge or discharge, or during battery operation in machinery or equipment, is too high, the unfavorable external temperature condition may cause the battery temperature to increase, leading to explosion or combustion of the battery.
Therefore, a rechargeable battery should pass various safety tests as to high temperature storage, thermal shock, temperature storage, and the like before it is distributed for sale. These safety tests are performed by leaving the battery in various temperature conditions for various lengths of time. Batteries should not explode or catch fire to pass the tests and may be unsealed to prevent explosion or combustion in extreme situations.
There have been various attempts to make secondary batteries safer. Particularly, much attention has been paid to technology for eliminating internal gas by equipping a safety vent on a battery case. Accordingly, a can-type battery using a prismatic or cylindrical can as a battery case is mounted with a safety vent having such a structure at a cap plate or at the can itself so that the battery cannot be destroyed when the internal temperature or pressure increases. However, unlike can-type batteries, pouch-type rechargeable batteries have not been able to be mounted with a safety vent, because their case is formed of a flexible material.